1. Field of the Invention
The present invention relates to plasma sputter/etching and more particularly to a high efficiency, low temperature system for growing or depositing various types of thin films on substrate surfaces, or etching such surfaces, using an improved hollow-cathode enhanced plasma at the substrate.
2. Prior Art
It is known that during plasma or RIE etching the use of a hollow-cathode electron source in the process significantly increases the presence of active etching species by increasing the electron density in the region of the etched surface so that the etching process takes place at a much faster rate or at a lower power level. See, for instance, U.S. Pat. No. 4,637,853 to B. Bumble et al and IBM TDB Vol. 28, No. 10, pps. 4294-7, March, 1986. The phenomenon of hollow-cathode discharge is explained in detail by C. M. Horwitz in Appl. Surf. Science 22/23, 925 (1985). Also, see his U.S. Pat. No. 4,521,286.
Enhanced growth of thin films is also possible with this phenomenon for the same reason and can be used to provide all the advantages to VLSI technology that low processing temperatures and high growth rate will offer. For example, reduced thermal exposure during oxidation can significantly reduce "bird's beak" which in turn will allow higher density circuits to be fabricated. In addition, less damage to the silicon/oxide interface will result due to the reduced power needed for this approach compared to other plasma systems.
One important area of film growth in VLSI technology is the production of hydrogenated amorphous silicon. Hydrogenated amorphous silicon (a-Si:H) has been intensively studied since Chittick et al achieved a dramatic reduction of the gap defect state density by using a glow discharge growth technique. See R. C. Chittick, J. H. Alexander, and H. F. Sterling, J. Electrochem. Soc. 116, 77 (1969). Subsequently, Spear and LeComber were able to substitutionally dope these films. See W. E. Spear and P. G. LeComber, Solid State Commun. 17, 1193 (1975) in this regard. Amorphous Si:H is now widely used in devices that require a large-area, and low-cost, low-temperature processing, e.g., solar cells, thin-film transistors for displays, electrophotography sensors, photodetectors, and light-emitting diodes. For a comprehensive review of this technology, see, for example, A. Madan and M. P. Shaw, The Physics and Applications of Amorphous Semiconductors (Academic Press, Boston, 1988), or J. I. Pankove, Ed., Hydrogenated Amorphous Silicon, in Semiconductors and Semimetals, volume 21 (Academic Press, Orlando, 1984).
As early as 1980, Knights concluded that optimal a-Si:H growth conditions led to a very low growth rate on the order of 0.5-1 .ANG./sec. See J. C. Knights, J. Non-Cryst. Solids 35/36, 159 (1980), and also A. Matsuda and K. Tanaka, J. Non-Cryst. Solids 97/98, 1367 (1987). A number of methods were suggested to increase the growth rate. For example, K. Ogawa, I. Shimizu and E. Inoue, in Japan. J. Appl. Phys. 20, L639 (1981), suggest the use of higher silanes as the silicon source gas. Very-high frequency rf glow discharge is discussed by H. Curtins, N. Wyrsch, M. Favre and A. V. Shah, in Plasma Chem. Plasma Process. 7, 267 (1987). The use of a grounded mesh peripherally surrounding parallel plate electrodes is described by T. Hamasaki, M. Ueda, A. Chayahara, M. Hirose, and Y. Osaka, in Appl. Phys. Lett. 44, 600 (1984). See also U.S. Pat. No. 4,633,809 to Hirose et al. Hydrogen-radical enhanced CVD, and hollow-cathode discharge are respectively suggested in Japan. J. Appl. Phys. 26, L10 (1987), by N. Shibata, K. Fukuda, H. Ohtoshi, J. Hanna, S. Oda, and I. Shimizu, and in the above-cited Appl. Surf. Science 22/23, 925 (1985) by C. M. Horwitz. Although optimal a-Si:H is usually deposited at temperatures between 200.degree. and 300.degree. C., deposition at lower temperatures was also attempted as described by Y. Ziegler, H. Curtins, J. Baumann and A. Shah, in Mat. Res. Soc. Symp. Proc. 149, 81 (1989) and references therein, and by G. Lucovsky, B. N. Davidson, G. N. Parsons, and C. Wang, in J. of Non-Cryst. Solids 114, 154 (1989). The motivation for achieving lowering temperature deposition was the ability to use heat sensitive materials as substrates. Furthermore, the discovery, reported by Z. E. Smith and S. Wagner, in Phys. Rev. B 32, 5510 (1985), that defects in a-Si:H participate in chemical equilibrium reactions, stimulated the study of the properties of samples deposited at lower temperatures (from room temperature up to 100.degree. C.). reported by R. A. Street and K. Winer, Mat. Res. Soc. Symp. Proc. 149, 131 (1989).
It is accordingly desirable in the art to have a high-efficiency system for depositing or growing films on, or etching, substrate surfaces, and particularly, to have, for example, an rf glow-discharge system for growing hydrogenated amorphous silicon (a-Si:H), at room temperature, on silicon.
The present invention improves upon prior art hollow-cathode-effect devices for sputter/etching by a technique which enhances plasma-produced growth or deposition of films on substrate surfaces, or the etching of such surfaces, and an apparatus design which will do so efficiently on silicon wafers. In a particular embodiment the present invention provides a new high-growth-rate technique for a-Si:H deposition.